Ductility and Brittle Fracture of Tungsten by Disconnection Pile-up on Twin Boundaries

Abstract

Refractory body-centered cubic (BCC) metals and alloys are of extraordinary importance in modern technological and structural applications. However, their wider adoption in science and technology is severely restricted by low-temperature brittleness, quantified by an unacceptably high value of the brittle-to ductile transition temperature (DBTT). The DBTT of these alloys is known to depend strongly on the particular microstructure of the material following mechanisms that are not well understood. Here we apply cross-scale molecular dynamics (MD), a simulation approach that preserves full atomic resolution while capturing the collective evolution of dislocations, twins, and cracks in near-micron-scale volumes, to investigate ductility and fracture in single-crystal tungsten pillars as a function of initial defect microstructure, deformation conditions, and temperature. The simulations reveal a sequence of microscopic processes conducive to failure: dislocation starvation, nucleation and growth of twins, pinning of the twin boundaries at surface asperities, resulting in disconnection pile-ups that trigger crack nucleation and propagation at low macroscopic stresses along incoherent boundary segments. By resolving these processes within a single atomistic framework, our simulations connect defect-level dynamics to macroscopic fracture behavior and identify microstructural pathways capable of shifting the DBTT through targeted promotion or suppression of the underlying deformation mechanisms.

Figures (10)

• Figure 1: Ductile behavior at lower temperatures and high straining rates was observed in simulations with periodic boundary conditions initiated with dislocation networks. The blocks can be deformed to essentially arbitrary strains (c) while maintaining constant dislocation density (b) and flow stress (a).
• Figure 2: Brittle behavior was observed at low temperatures in simulations with surfaces. The initial dislocation starvation stage (d.1,d.2) leads to increasing stress (a) and twin nucleation (d.3). While the twin growth initially results in a ductile deformation, eventually the moving twin boundaries get pinned by surface imperfections resulting in a disconnection pile-up near the surface pinning site. This pile-up leads to the formation of an inclined, incoherent twin boundary segment, which serves as a critical site for crack nucleation and propagation (d.4-d.6) at low macroscopic stress (a).
• Figure 3: Crack nucleation mechanism by disconnection pile-up at twin boundaries. (a) A twin formed by two planar coherent twin boundaries that migrate during twin growth by nucleation and propagation of disconnections. Snapshots (a.1) and (a.2) show the same atomistic TB configuration viewed at different scales. (b) Early stage of twin boundary pinning with a single disconnection stuck near the surface roughness. (c) Continuous TB migration results in a disconnection pile-up and the formation of a longer inclined boundary segment. (d) Crack nucleation and propagation through the disconnection pile-up region. (e) Schematic illustrating the disconnection pile-up mechanism.
• Figure 4: The ductile-to-brittle transition temperature (DBTT) is predicted by simulations with pre-existing dislocations and free surfaces. (a) The DBTT is marked by the absence of twin boundary pinning and disconnection pile-up. (b) The stress-strain curves show brittle fracture at low temperatures and ductile behavior at high temperatures. While the two initial stages, including dislocation starvation and twin nucleation, occur at all temperatures, high-temperature samples continue to deform in a ductile manner through dislocation nucleation after twin boundaries migrate through the sample without pinning and eventually annihilate.
• Figure 5: Ductile deformation of samples with lateral free surfaces at high temperatures. Although the deformation proceeds through the same stages of starvation (a2) and twinning (a3), at higher temperatures twins do not get pinned by surface defects and no fracture occurs. Twin propagation proceeds in a ductile manner until the two TBs annihilate with each other across the periodic boundary conditions. The subsequent deformation proceeds by nucleation of dislocations at surfaces which eventually leads to gradual necking (a4). (b-d) Stress, dislocation density, and TBs atoms as a function of strain reflecting the three different deformation stages. The samples were deformed at a strain rate of $10^7$ s$^{-1}$ at 2000 K.